A backlight module of a large extra-thin or a high brightness extra-thin liquid crystal display needs a large amount of light-emitting diodes resulting in a larger wiring area being required for arranging the wires of the light-emitting diodes. Currently, the wiring area of the flexible circuit board of the light-emitting diode light bar is increased typically by increasing the width of the flexible circuit board or by using a multi-layered flexible circuit board.
FIG. 1 illustrates a cross-sectional view of a portion of a conventional backlight module. A backlight module 100a comprises a reflective cover 102, a light guide plate 110 and a light-emitting diode light bar 104, wherein the light-emitting diode light bar 104 is composed of a flexible circuit board 108 and many light-emitting diodes 106 disposed thereon. In the backlight module 100a, in order to provide the light-emitting diodes 106 with a sufficient wiring area, the width w of the flexible circuit board 108 of the light-emitting diode light bar 104 is increased. In the backlight module 100a, the light-emitting diode light bar 104 is disposed in the reflective cover 102, the flexible circuit board 108 extends towards the outside of the reflective cover 102, and the light guide plate 110 is disposed between the upper side plate and the lower side plate of the reflective cover 102, wherein the light guide plate 110 and the reflective cover 102 surround all of the light-emitting diodes 106.
In the backlight module 100a, the width w of the flexible circuit board 108 is sufficiently large to enable stacking the light guide plate 110 on a portion of the flexible circuit board 108. The flexible circuit board 108 has a thickness d, so that the thickness of the backlight module 100a is increased, therefore it cannot fulfill the requirement for the liquid crystal display product, which has a strict requirement of thickness. FIG. 2 depicts a backlight module 100b, in order to solve the issue that the width w of the flexible circuit board 108 of the light-emitting diode light bar is too wide, the flexible circuit board 108 is bent to an L-shape, and the flexible circuit board 108 is adhered to the two adjacent side plates of the reflective cover 102. However, in a larger size or higher brightness liquid crystal display, the flexible circuit board 108 located on the lower side plate of the reflective cover 102 is still too wide, so that the thickness problem remains.
FIG. 3. depicts a backlight module 100c in which, in order to further improve the issue that the width w of the flexible circuit board 108 of the light-emitting diode light bar is too wide, the flexible circuit board 108 is bent to a ninety degree rotated-U shape, and the flexible circuit board 108 is adhered to three side plates of the reflective cover 102. However, the U-shaped flexible circuit board 108 greatly increases the fabrication difficulty.
FIG. 4 illustrates a cross-sectional view of a portion of another conventional backlight module 100d. A light-emitting diode light bar 114 of backlight module 100d adopts a flexible circuit board 112 with multi-layered circuits to narrow the width of the flexible circuit board 112 in comparison to flexible circuit board 108 described above, so the light-emitting diode light bar 114 can be entirely located in the space surrounded by the reflective cover 102 and the light guide plate 110. However, the thickness of the multi-layered flexible circuit board 112 is greatly increased, and the thermal conductivity of the insulation layer between the two adjacent layers in the flexible circuit board 112 is poor, so that the heat generated by the light-emitting diodes 106 of the light-emitting diode light bar 114 cannot be effectively conducted, thereby decreasing the efficiency, shifting the shades of color and reducing the life of the light-emitting diodes 106. In addition, the multi-layered flexible circuit board 112 is expensive, so that using the flexible circuit board 112 greatly increases the cost of the backlight module 100d. 